When images are digitized, the image information is usually encoded and compressed. An effective method for compressing the image information is to reduce the spatial redundancy contained in the image. These methods typically use the well-known discrete cosine transform (DCT). The input image data are divided into blocks of, for example, 8 pixels by 8 lines. A 2-dimensional DCT is applied to generate DCT transform coefficients with the correlation between pixels removed. Usually, these coefficients are quantized to integer values with finite precision (bit length) and then converted into a bit sequence of binary codes employing variable length encoding such as Huffman codes.
When a moving picture is digitized, temporal information compression is also carried out in addition to spatial information compression. In temporal information compression, the inter-frame difference between the current frame and the previous frame is encoded. Inter-frame prediction encoding with compensation for the moving portions are often adopted. In inter-frame difference encoding, a DCT of the difference image represents the difference between the current frame and the previous frame.
Usually, a prescribed standard is defined for the bit rate for sending the encoded data to the transmission line from the encoder that performs said image information compression taking into consideration the capacity of the transmission line and the characteristics of the decoder. The bit rate of the encoding output by the encoder is monitored based on the selected standard and the quantity of code generated is feedback-controlled. For example, when the moving images are transmitted at a frame rate of 15 frames/sec, the quantizing coefficients are adjusted and the code quantity for each frame is variably controlled so that it conforms to the bit rate standard for the entirety of 15 consecutive frames during the period of 1 sec. However, when a scene change takes place or when there is a significant change in the image, a huge quantity of information is instantly generated. Consequently, an insufficient quantity of code may be allotted to the later frames. This may result in a so-called frame omission. The number of frames per second must be reduced, significantly degrading the image quality. This is undesirable.